Serially concatenated continuous phase modulation (SC-CPM) systems combine the benefits of a constant envelope signal, excellent spectral properties, and good performance with iterative demodulation/decoding. When used over frequency-selective fading channels with long delay spreads, however, a large number of states in the joint CPM-channel trellis makes the trellis-based demodulation/equalization prohibitively complex. In this paper, we overcome this problem by employing a new reduced-complexity soft-input soft-output (SISO) algorithm, the M{sup}*-BCJR algorithm. The performance/complexity tradeoffs involved in the system design are investigated and the M{sup}*-BCJR system is compared to the BCJR and Reduced State BCJR systems by means of simulations.
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